25-08-2017, 09:32 PM
Spread Spectrum
Spread Spectrum[.ppt (Size: 223.5 KB / Downloads: 12)
Introduction to Spread Spectrum
Problems such as capacity limits, propagation effects, synchronization occur with wireless systems
Spread spectrum modulation spreads out the modulated signal bandwidth so it is much greater than the message bandwidth
Independent code(PN code) spreads signal at transmitter and dispreads signal at receiver
Frequency multiplex
Separation of spectrum into smaller frequency bands
Channel gets band of the spectrum for the whole time
Advantages:
no dynamic coordination needed
works also for analog signals
Disadvantages:
waste of bandwidth
if traffic distributed unevenly
inflexible
guard spaces
Spread Spectrum Technology
Side effects:
coexistence of several signals without dynamic coordination
tap-proof
Alternatives: Direct Sequence (DS/SS), Frequency Hopping (FH/SS)
Spread spectrum increases BW of message signal by a factor N, Processing Gain
FH/SS (Frequency Hopping Spread Spectrum) I
Discrete changes of carrier frequency
sequence of frequency changes determined via PN sequence
Two versions
Fast Hopping: several frequencies per user bit (FFH)
Slow Hopping: several user bits per frequency (SFH)
Advantages
frequency selective fading and interference limited to short period
uses only small portion of spectrum at any time
Disadvantages
not as robust as DS/SS
simpler to detect
Performance of DS/SS Systems
Pseudonoise (PN) codes
Spread signal at the transmitter
Despread signal at the receiver
Ideal PN sequences should be
Orthogonal (no interference)
Random (security)
Autocorrelation similar to white noise (high at =0 and low for not equal 0)